Elasticity events goodput logs

[dipannita08]

Description

Adding three types of elastic event goodput logs using the Custom Badput APIs:

• elastic_scale_up
• elastic_slice_down
• elastic_reinitialization

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• why is this change being made,
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FIXES: b/123456
FIXES: #123456

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Tests

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Checklist

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[codecov]

Codecov Report

❌ Patch coverage is 4.00000% with 24 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/maxtext/trainers/pre_train/train.py	0.00%	24 Missing ⚠️

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